This invention relates to a resin-molded semiconductor device, and more particularly to improvements for preventing the concentration of stresses on the plastic package of the semiconductor device.
As a packaging structure for resin-molded semiconductor devices, especially power transistors and power ICs, the inventors of the present invention have proposed a structure as shown in FIGS. 1 and 2 wherein a semiconductor element (chip) 2 is mounted on the surface of a heat radiating base plate (header) 1 made of an electric conductor such as copper and is sealed by a plastic package 3 of an epoxy resin or the like in a manner to be encapsulated therein (for the sake of clarity, leads etc. are omitted in FIGS. 1 and 2). The heat radiating base plate 1 has a screw mounting portion (external connection portion) 4 which serves to bring the semiconductor device into touch with an external base plate (external heat sink) of a metal or the like, and which is exposed outside the plastic package 3. The mounting portion 4 has screw holes 5. The lateral width X.sub.1 of this portion 4 is greater than the lateral width X.sub.2 of a mounting portion 14 for the semiconductor element (chip) 2. The rear surface of the semiconductor element-mounting portion 14 is exposed outside the plastic package 3. Stepped out-away portion 6 are formed between the screw mounting portion 4 and the semiconductor element-mounting portion 14. The plastic package 3 for encapsulating the semiconductor element 2 therein seals the element in contact with the side surfaces of the cut-away portions 6. According to the structure as described above, the element mounting portion 14 of the metallic base plate 1 to be sealed in the plastic package 3 is smaller in width than the plastic package 3, and hence, the amount of warp of the plastic package 3 ascribable to the difference of the coefficients of thermal expansion of the metallic base plate 1 and the plastic package 3 can be made small during the resin-molding. In addition, since the external connection portion 4 has the width greater than that of the element mounting portion 14, the semiconductor device can be rigidly attached to the package mounting base plate or heat sink and also good characteristics of heat radiation can be attained.
The inventors of the present invention, however, have found that a disadvantage to be described hereunder is involved in the resin-molded semiconductor device of the above structure. That is, in a case where the radiating base plate 1 was attached to the package mounting base plate or heat sink 13 by a screw 15 as illustrated in FIG. 2, stresses F acted on the side surface part of the radiating base plate 1 upon tightening the screw, and the stresses were especially concentrated on a sharp corner 7 in the cut-away portion 6. In consequence, resin cracks 8 developed in that part of the resin molding member 3 which lay in contact with the side surface of the radiating base plate 1 and which was close to the corner 7 of the cut-away portion (refer to FIG. 3). Accordingly, the airtightness of the package was spoiled, and deteriorations in the characteristics of the sealed semiconductor element were incurred. Particularly when the package mounting base plate 13 was not flat, this phenomenon occurred conspicuously.